Following the IgE-mediated activation of mast cells, and the immediate allergic response, lymphocytes enter the inflammatory site, and are believed to secrete cytokines that promote the late inflammatory response, or late phase reaction. The mast cell derived molecules that promote lymphocyte chemotaxis are largely unknown. We have thus recently focused on this step in the allergic inflammatory response and have identified both a chemokine (lymphotactin [Ltn] and a cytokine (IL-16) produced by mast cells that may contribute to the ingress of lymphocytes, and the stimuli that regulate their synthesis and release. Using RT-PCR and Northern blot analysis, we found that the Ltn gene is inducible in C1.MC/C57.1 and murine bone marrow-cultured mast cells (BMCMC) by FceRI aggregation. Activation of a human mast cell or basophil cell line similarly led to transcription of Ltn. FceRI aggregation-dependent Ltn mRNA expression was maximal at 6 h, independent of de novo protein synthesis, and was inhibited by cyclosporin A and dexamethasone. Compared with macrophage inflammatory protein alpha (MIP- 1a), FceRI-dependent Ltn and MIP-1a mRNA levels were up-regulated by IL-4, but not IFN-g, although higher levels of IL-3 inhibited Ltn expression only. TGF-b preferentially enhanced FceRI-dependent Ltn mRNA levels. A rabbit polyclonal Ab against a synthetic peptide was developed for use in immunoblot analysis and detected a 15-kDa Ltn protein within mast cell pellets and in supernatants of mast cells following FceRI aggregation. Similarly, IL-16 is stored preformed in bone marrow cultured human mast cells (BMCMC) and a human mast cell line, as demonstrated by intracytoplasmic cytokine staining and flow cytometry; and in human lung mast cells as detected by immunohistochemistry. In response to the anaphylatoxin, C5a, or PMA treatment, IL-16 mRNA transcripts detected by Northern blot analysis in HMC-1 cells increased 6 to 10 fold. HMC-1 cell lysates and activated supernatants contained IL-16 protein as demonstrated by both ELISA and in vitro lymphocyte chemotaxis assays, the later of which was blocked 59-88% by the addition of neutralizing antibody to recombinant human IL-16. IL-16 bioactivity was detected in the supernatants 2 to 4 h after PMA or C5a activation, and this activity remained elevated through 24 h. The capacity of mast to thus synthesize and release Ltn and IL-16 provides a possible link between mast cell activation and the accumulation of T cells in mast cell-dependent inflammation, thus amplifying the immune response and perpetuating the pathological process. The expression of novel genes in mast cells after activation is also being examined by means of screening of activation cDNA libraries. Some of these clones isolated represented genes that are known to be upregulated in mast cells after activation, such as Ltn and MIP-la. Several of the clones represent new genes. The expression of these genes is being examined by Northern blot analysis and the genes characterized.